Potentiometer



June 26, 1951 w. D. VAN DYKE POTENTIOMETER 2 Sheets-Shea?. 2

Filed Oct. 19, 1949 WML/.4M a. VAN orme',

JNVENTOR. BY///Q/ ATTORNEY.

Patented June 26, 1951 UNITED STATES nPATENT OFFICE POTENTIOMETERWilliam D. Van Dyke, St. Petersburg, Fla.

Application October 19, 1949, Serial No. 122,205

" y defects or errors in the use of the potentiometer Potentiometers ofthe type herein shown and jL described have been previously proposed andbecause of the helical arrangement of the resistance element thesepotentiometers are being used in many iields where extremely delicatemeasurevments are required. They are especially adapted to elds in whichlow torque instruments are required, as the very high resolution permitsthe use of a contact member extremely light in weight. As very lightcontact members could be used, it was of course relatively easy todesign actuators for moving the contact members which could be operatedwith relatively small forces. These prior low torque designs, however,created a problem in that it was diiiicult to accurately limit movementof the sliding Contact between predetermined positions at opposite endsof the resistance element.

This was so as the very light sliding contacts were not rugged enough totake the forces exerted Y relatively thin disc of insulation material.

even in aircraft or other flight instrumentalities, such as rockets.

The resistance element of the potentiometer herein shown and describedis helical in form and is engaged by a sliding contact carried by a Thedisc is keyed to a shaft which is mounted for` rotation about an axiscoincident with the longitudinal axis of the resistance eiement. Thedisc also carries means extending between adjacent or acting thereon ifthe contact member itself made to obviate this difficulty, none havesucl cessfully solved the problem created. These proposals have in mostpart failed because they have attempted to use the Contact member itselfas a part of the stop mechanism or as a means for Y actuating the stopmechanism. Where some success has been had by strengthening the contactmember or its carrier, it has been gained at the expenseof the lowtorque operational feature of the potentiometer. Other proposalsinvolved the employment of mechanisms which not only produced hysteresiswhen the potentiometer was connected to a low torque actuator, but alsointroduced errors resulting from elements of the mechanism adverselyaffected by acceleration, deceleration and other conditions encounteredin some fields in which such potentiometer-s are used today.

The potentiometer of the present invention is free of the objectionsfound in these prior potentiometers, for the same can be used with anylow torque actuator, is free of hysteresis, and the limit stops,positive in action, are o1" such construction that they do not introduceoperational resistance element.

coils of the resistance element and cooperative therewith as the shaftis rotated, to move the disc longitudinally of the shaft as it rotatestherewith to move the Contact along the helical The disc and contactmember are extremely light in weight and thus do not produce inertiaforces which would introduce hysteresis when low torque actuators areused to operate the potentiometer. It is possible to use verylightweight elements in the potentiometer of the present invention forthe limit stops of the potentiometer are not a part of the contactmechanism, but are formed as separate means.

The means used to form the limit stops of the potentiometer of thepresent invention are located on the shaft to which the contact carrieris mounted and the restraining forces are taken not by the carrier orthe contact member thereon but by the shaft itself. The stop meansincludes, in the one embodiment of the invention illustrated herein, ahelical spring having the same number of turns as the resistanceelement. The spring sleeves the shaft and co-acts with a small lug tomove the latter longitudinally of the shaft as the same is rotated. Apair of abutments, xedly carried by the shaft and longitudinally spacedapart thereon, limit longitudinal movement of the lug in oppositedirections for the helical spring extends between the abutments and theturns of the spring determine the number of rotations of the shaftnecessary to move the lug from a position in which it is engaging one ofthe abutments to a position in which it is engaging the other of saidabutments.

Movement of the lug in either direction along the shaft by rotation ofthe latter is stopped as soon as the lug is moved into engagement withone or the other of said abutments. As the lug is solidly mounted in akeyway or slot of a journal member supporting the shaft for rotation,the shaft is also held against rotation once the lug is moved intoengagement with an abutment.

It will thus be seen that the stop mechanism of the potentiometer of thepresent invention is positive in action and is actually formed in partby the shaft itself. As the shaft can be formed to take relatively hightorsional forces without increasing the torque required to producerotation of the shaft and consequently movement of the contact member,the stop mechanism is extremely rugged and yet does not impair the loWtorque operation of the potentiometer.

The stop mechanism does not complicate assembly of the potentiometer,for the resistance element is merely mounted in its housing in such away that the element, when mounted, will remain fixed relative to thehousing and the stop mechanism then assembled in such a manner that itwill limit movement of the sliding contact member between predeterminedpositions at opposite ends of the element. This is so, for the helicalspring is so formed that the coils thereof dei-lne a helical passagewayof the identical number of turns as the helical resistance member andthe position of the lug relative to the coils or turns of the spring canbe set to `always match the position of the contact relative to thecoils or turns of the resistance element as the shaft is rotated.

Another feature of the potentiometer of the present invention resides inthe particular coaction between the resistance element and the stopmechanism. As the helical spring producing longitudinal movement of thelug between the abutment also determines the number of cornpleterevolutions of the shaft necessary to move the contact about the entireseries of turns of the resistance element, it is obvious that the samestop mechanism is easily converted for use with resistance elements ofvarying numbers of turns by merely using a spring defining a passagewayof the same number of turns as the particular element with which thestop mechanism is used. For example, if the potentiometer is a ten-turninstrument, i. e., has a resistance element formed of ten turns, thespring must dene a passageway of ten turns, and by merely substituting aspring which defines an eight-turn passageway the stop mechanismfunctions equally as weil with an eight-turn resistance element.

In another embodiment of the present invention, the stop mechanism issubstantially the same as above mentioned except that two lugs are used.In this form of the invention, the resistance element is formed of arelatively small number of turns and the helical spring controllingmovement of the lug is formed with a larger number of turns than theelement. In the assembly of this form of the stop mechanism, the one lugis so disposed as to be engaging one abutment while the other lug ismounted between the turn of the spring corresponding to the maximumnumber of turns of the element. Thus, when the shaft is rotated thefirst lug moves out of engagement with its abutment and the second lugwill move into engagement with the other abutment after the shaft hasbeen rotated a number of revolutions equal to the number of turns of theelement.

Thus, the potentiometer of the present invention has all of thedesirable features of the previously proposed helical potentiometers inthat the elongate resistance element permits Very accurate measurementsto be made. Furthermore, the contact mechanism is such that low torqueactuators can be used without the danger of hysteresis creating errorsin the measurement affected. The stop mechanism, separate from thecontact mechanism, is extremely rugged and,

while positive in action, does not complicate either the assembly of theinstrument or its operation.

Other features and advantages of the present invention will be apparentfrom the following detailed description, particularly when taken inconnection with the accompanying drawing, in which:

Figure l is a sectional view of one embodiment of the potentiometer ofthe present invention including the registering actuator for rotatingthe shaft thereof;

Figure 2 is a section taken along line 2--2 of Figure 1;

Figure 3 is a perspective view of the contact member coaxially carriedby the shaft and shown detached from the latter;

Figure i is a perspective view of the registering actuator for rotatingthe shaft;

Figure 5 is aperspective View of the contact carrier shown detached fromthe shaft;

Figure 6 is a section taken along line 6-6 of Figure l;

Figure 7 is a sectional view of the potentiometer showing the lug inengagement with one of the abutments;

Figure 8 is a perspective view of the lining sheet which is bonded aboutthe resistance element with the ends of the lining sheet separated tomore clearly illustrate the leads of the resistance wire; and

Figure 9 is a view similar to Figure l showing a modified form of thepotentiometer.

The potentiometer of the present invention, referring now to thedrawing, and more particularly to Figure l thereof, comprises aresistance element iii, consisting of a core member II, formed as ahelix, having, in the embodiment of the invention illustrated, ten turnsor coils. The core member I I carries the resistance wire, which isnelically wound thereon as in a conventional potentiometer with theopposite-ends of the core exposed or free of resistance wire. Theresistance wire is insulated from the core member preferably by coatingthe core member with a suitable insulating material prior to the step ofwinding the resistance wire thereon.

In the now preferred method of forming the resistance element, the same,after the resistance wire is wound about the core II, is dipped into aliquid bath of some suitable insulating material, which when set andhardened not only insulates the turns of the resistance wire from eachother, but also holds these turns against displacement. After thematerial in which the element is dipped has set and hardened, the coremember is formed into a helix having the number of turns desired. Theinsulation material is now removed from at least the inner surfaces ofthe turns of the element to permit electrical engagement between theresistance wire and the movable contact I2 of the potentiometer.

In the illustrated embodiment of the present invention, the movablecontact I2 iscarried adjacent the one end of a spring arm I3 theoppositeend of which is fixed to a disc I4 formed of some non-current conductingmaterial. The disc I4, referring now to Figure 5, is formed with acentrally located aperture I5 to permit the disc to be coaXially mountedto a shaft I6 adapted to rotate about an aXis coincident with the axisof the resistance element. The disc I4 is also formed with a narrow,radially extending slot I'I opening at one end into theaperture I5 forpassing a key I8 longitudinally extending along the S shaft I 8. Itshould now be seen that the disc Il because of the key and slotconnection to the shaft I6 will rotate therewith, but yet is movablelongitudinally of the shaft I6.

To move the disc Il longitudinally of the shaft I6, the disc, in theillustrated embodiment of the present invention, is split as indicatedat i9 and the adjacent ends of the split portion of the disc are heldapart by the free end of the spring arm I3 to which the contact member jI2 is mounted. it Will be seen now that the peripheral portion of thedisc forms a helix and the helically arranged peripheral portion of thedisc extends between adjacent turns of the resistance element I0. theresistance element I3 is fixed within the casing 2l, and as the shaft l'turns relative to the casing 2I, the peripheral portion of the disc I4extending between adjacent turns of the element acts as a follower whichis cammed to the left or right as viewed in Figure l, depending upon thedirection or rotation of the shaft I6. lt should be understood that theturns of the resistance element I will act as a cam, coacting with theperipheral portion of the disc I4 to move the latter longitudinally ofthe shaft ni as the same is rotated. This movement of the disc lfl willof course cause the contact member I2 to move around the individualturns or coils of the resistance element to introduce varying c degreesof the resistance into a circuit forming a part of the controlapparatus.

The casing 2l in which the resistance element I?) is fixed includes acylindrical element 22, the

opposite open ends of which are closed by disc :r

members 23 and 2e respectively. The cylindrical member 22 of the casing2l is provided with a lining element of non-current conducting material,which may comprise a sheet of some suitable paper, which preferably iscoated i i or impregnated with a resinous material to strengthen thesame, as well as to render the paper impervious to moisture. The sheetof paper is somewhat longer than the circumference of the innercylindrical Wall of the casing 2l so that the opposite ends thereofoverlap after the lininfT sheet dis coiled into a cylindrical form. Thisis clearly shown in Figure 2 of the drawing.

The resistance element, whose outer diameter is substantially the sameas the inner diameter of the lining 25, is securely anchored or bondedto the latter by a bonding agent indicated at The bonding agent mayconsist of the material used to impregnate the lining sheet, or may be aseparate material applied between the impregnated lining and the turnsof the resistance element. Whatever material is used, the same should beone which will securely hond the turns of the element to the liningsheet.

The lining sheet 25 is held against rotation relative to the casing 2lby the drive screws 2l, or other fastening elements, used to mount thediscs 23 and within the opposite ends of the cylindrical member 22. Asthe lining 25 is held against rotation relative to the casing, theresistance element is, of course, also held against movement relative tothe casing.

The disc 23 carries the terminals of the potentiometer and the rightwardend of the resistance element Ill, as viewed in Figure l, is connectedto a terminal 23 by a lead 29 electrically connected to the one end ofthe resistance element and, as best seen in Figure 8, passed between theoverlapping end portion of the lining 25 somewhat longitudinally of thecasing to a As will be shown,

small aperture formed in the inner overlapping end portion adjacent tothe terminal 28. The lead 29 is passed into the interior of the housingthrough this aperture and then lead outwardly of the housing through apassage 30 in the disc 23 formed adjacent the terminal 28 which is fixedin an opening in the disc 23 by peening over the inner end thereof asindicated at 3I. The free end of the lead 29 is preferably secured to apin 32, carried by the terminal 28, by a solder, although other meansmay be used to electrically interconnect the lead 29 and the pin 32.

A lead 33 electrically connected to the left ward end of the resistanceelement, as viewed in Figure l, is passed through an opening formed inthe inner overlapping end portion of the lining 25, referring again toFigure 8, and then passed longitudinally to the left, as viewed inFigure 1, to a second aperture through which the lead is passed into theinterior of the casing 2l. The free end of the lead is then passedthrough a passage 34 in the disc 23 contiguous to a terminal 35 similarto the terminal 28 which is xed to the disc 23 in the same manner as isterminal 28 xed to the disc 23. The lead 33 is electrically connected tothe pin of terminal 35 in the same manner that lead 23 was fixed to thepin 32 of terminal 28.

A flexible lead 33 of a length substantially equal to the shaft I iswrapped about the latter and has one end electrically connected to theresilient arm I3 which carries the contact member I2. The opposite endof this lead is electrically connected, referring now to Figure 3, to anannular member 3l of a resilient current conducting material carrying acontact member 38, The member 3'! is fixed to a disc 39 of some suitableinsulation material, which is coaxially mounted to the one end of theshaft I5 and rotates therewith. As the conducting member 3'! is fixed tothe disc 39, the contact member 38 obviously will movel in an annularpath as the shaft I5 is rotated. The resilient current conducting member3l holds the contact 32 carried thereby in good electrical engagementwith an annular contact 4I xed to the inner face of the disc 23 by aterminal member ill similar to members 28 and 35, and which includes apin 45 outwardly projecting from the disc 23.

It should now be seen that as the shaft IG is rotated and the disc Illis moved longitudinally thereof, the flexible lead 35 will follow thedisc I4 to the end that the contact l2 is at all times electricallyconnected to the pin 55. Care should be taken to insure that theresilient member 3l' to which the contact 38 is mounted will always urgethe latter against the fixed contact di to maintain good electricalengagement between the contact 38 and the annular contact member -I. Thepotentiometer just described has all of l the desirable features ofpreviously proposed potentiometers of this type, for the elongateresistance element I9 permits, as should be understood, extremelyaccurate measurements to be made. As the disc M to which the contact I2is mounted is formed of a relatively light weight material, and as thisdisc itself is employed to move the contact I2 longitudinally of theresistance element Il), there is little resistance to movement of thecontact I2, and the shaft I6 can vbe connected to measuring instrumentsof vari-J ous types, which develop relatively low torque forces inoperation.

The potentiometer of the present invention is therefore not likely toproduce errors in measurement due to hysteresis, In fact it has beenfound that in some elds of use, it is necessary to introduce somefrictional resistance to rotational movement of the shaft I5, and thisis most conveniently done by mounting a spring washer 46 between theinner face of the disc 24. and a collar 41 fixed to the shaft I6adjacent to its passage through the disc 2d.

It is of course desirable to provide some mechanism for limiting themovement of the contact I?. along the element Ic and between the exposedopposite ends of the same, as well as for preventing the disc I5 frombeing moved out of engagement with the outer turns of the resistanceelement I for it will be remembered that it is this engagement whichproduces the longitudinal movement of the disc I relative to the shaftIS. Although in some previously proposed potentiometers, this stopmechanism has coacted with the contact carrier itself to limit movementof the latter, in the present invention the stop mechanism is separateand forms no part of the contact carrier. This is done to relieve thedisc I4 of the strain which would obviously be irnposed thereon if thedisc was moved into engagement with xed abutments or the like, whichwould limit movement of the disc I5.

The stop mechanism of the potentiometer of the present invention isformed by reducing the shaft I6 as indicated at 4S to form an annularchannel receiving a helical spring 59, rThe spring circumscribes orsleeves the bottom wall of the channel and the opposite ends of thespring are fixed to the shaft I5 by anchoring the ends in narrow groovesformed adjacent the side walls 5I and 55E of the channel. The side walls5I and 52 of the channel are formed with abutments 53 and 5d,respectively, each having a relatively wide face extending substantiallyparallel to the longitudinal axis of the shaft i6. The abutments arearranged in oppositely facing relation and, as will be hereinaftershown, cooperate with a small lug 55 to limit rotation of the shaft i6and consequently movement of the contact I2.

The lug 55 is slidably mounted in a slot 55 formed in the wall of thebore of an elongate bearing member '5l in which the shaft I5 isjournalled The bearing member' 5'! is formed with an enlarged head-likeextremity which is coaxially fixed in a centrally located opening in thedisc with the body or shank portion of the bearing member projectingoutwardly from the exterior face of the disc. It will be seen that theslot 55 extends longitudinally of the shaft l5 and is of a depth lessthan the thickness of the lug 55 so that a portion of the lug projectsinto the channel 48. The outer face of the lug 55 actually engages thebottom wall of the channel 48 and is thus supported for sliding movementby the walls of the slot 55 as well as the bottom wall of the channel5S. The portion of the lug 55 projecting from the groove 55 is adaptedto be mounted between adjacent turns of the spring 49 which, as theshaft is rotated, moves the ylug 55 longitudinally of the slot 55 in onedirection or the other depending upon the direction of rotation of theshaft I5. The coils of the spring rie disposed between the side Walls 5Iand 52 are of such a number that the same define therebetween a helicalpassageway of the same number of turns as the resistance element. Thehelical passageway den'ed bythe coils of the spring extends from oneabutment to theother and the lug moves relatively 'throughthe'pasgeraete 8 Sagevvay as the spring revolves with the shaft. Thusrevolutions of the shaft equal in number to the-turns of the resistanceelement will move the lug from one abutment to the other.

As long as the lug is between any adjacent turns of the spring Il@ otherthan the outer pairs of adjacent turns, the lug will be movedlongitudinally of the shaft I5, for each rotation of the same, adistance equal to the diameter of the wire of which the spring isformed. The lug during its relative movement around and through theouter pairs of adjacent turns of vthe spring must move a relativelygreater distance, however, to bring the lug into facewise engagementwith one or the other of the abutments 53 and 54.

To bring about this movement the side walls 5I and 52 of the channel areformed as helical surfaces having a pitch substantially equal to thewidth of the faces of the abutments 53 and 54. With the side walls 5Iand 52 thus formed, the lug during its relative movement through theouter pairs of adjacent turns of the spring will move a distancelongitudinally of the shaft equal to the pitch of the helical surfacespresented by the walls 5l and 52 of the channel 4B. This is so, for thelug, as it distends the turns of the spring 45, will be urged therebyagainst one or the other of the helical surfaces presented by the sidewalls 5I or 52 and consequently when disposed between either of theouter pairs of adjacent turns will move during a complete revolution ofthe shaft l5 a distance substantially equal to the pitch of thesurfaces, which, as above explained, is substantially equal to thew-idth of the faces of the abutments. Movement of the lug intoengagement with either abutment will, of course, hold the shaft againstrotation.

This construction results in a more positive and more rugged stopmechanism, for the lug can be formed of a relatively larger size thanwould be possible if the helical surfaces were not present. This shouldbe obvious, for otherwise v the abutment could be no wider than thediameter of the wire of which the spring is formed,

In the assembly of the potentiometer of the present invention thecontact member "i 2 is positioned so that it is in engagement with apredetermined point or position at the one end of the resistance elementI0. While the contact member is held in this position the stop mechanismis assembled with the lug in engagement with the abutment 54. With thepotentiometer 'thus assembled, rotational movement of the shaft I6 will,as previously explained, move the disc I4 longitudinally of the shaft tobring about movement of the contact member I2 around the turns of theresistance element Ill. With the potentiometer assembled as abovedescribed, that is, with the lug 55 engaging the abutment 54 and thecontact I2 engaging a predetermined position at the rightward end of theresistance element as viewed in Figure l, ten turns of the shaft I5 willmove the lug into engagement with the abutment 5c as the contact I2moves into engagement with a predetermined position at the opposite orleftward end of the resistance element as viewed in Figure l.

Although any means may be used to turn or rotate the shaft I6, in theembodiment of the invention illustrated in Figure l, a registeringactuator 58 of the type shown and disclosed in my copending application,Serial No. 790,402, (now Patent No. 2,532,970, issued December 5a 1950)is shown. This actuator comprises a body member 59 directly fixed to theone end of the shaft I6 and consequently the shaft is rotated as thevbody member is rotated. The manner in which the elements of theactuator cooperate with each other to register turns of the shaft I6 isnot required for a full understanding of the invention herein shown, butsuffice it to say, the actuator comprises a pair of scales 60 and 6I,referring now to Figure 4, which indicate, respectively, fractional andcomplete revolutions of the shaft I6.

Although the lug is subject to shear forces, if an attempt is made torotate the shaft after the lug is engaged with one or the other of theabutments, by merely forming the lug of a material having a high shearstrength, all danger of the lug failing in use is eliminated. The stopmechanism of the Apresent invention furthermore is positive in action,for the retaining forces are exerted directly on the shaft and notindirectly as in some prior potentiometers. The shaft, however, is wellable to take the torsional forces created therein as the lug is movedinto engagement with one or the other of the abutments integrally formedon the shaft. Furthermore, the stop mechanism of the present inventionin no way interferes with the operation of the potentiometer and doesnot increase the torque required to rotate the shaft I5.

As the turns of the helical spring control the relative movement of thelug about the shaft in substantially the same way as the turns of theresistance element control the relative movement of the contact relativeto the shaft, a potentiometer having a five-turn resistance element, forexample, can be provided with a stop mechanism of the present inventionby merely using a five turn helical spring.

There is shown in Figure 9 a potentiometer of substantially the sameconstruction as that of the potentiometer shown in Figure 1, and inwhich elements thereof identical to elements of the first describedpotentiometer are identified by the same reference characters. Thepotentiometer of Figure 9 differs from the potentiometer shown by Figure1 only in the number of turns of the resistance element. The resistanceelement e2 of the potentiometer illus trated in Figure 9 is formed withbut ve turns and consequently can be housed in a casing 63 of a shorterlength than the casing 2|.

As the resistance element 62 has but five turns, the spring 64controlling movement of the lug 1 55 is consequently formed of thenumber of turns or coils which will define a five turn passagewaylthrough which the lug relatively moves as the shaft is rotated. Themode of operation of the potentiometer of Figure 9, however, is the sameas that of the potentiometer shown by Figure l, with this exception: Thecontact I 2 is moved from one extreme position at 'the one end of theresistance element to a second extreme position at the opposite end ofthe element by five complete revolutions of the shaft 65. The fivecomplete revolutions of the shaft also, as should now be understood,move the lug 55 from e, position in which it is engaging the oneabutment to a second extreme position in which it is engaging the secondabutment. Thus, here again the lug moves into engagement with anabutment and prevents further rotation of the shaft at the same time thecontact moves into its limit or preselected position on the resistanceelement.

It should be seen now that by merely changing the helical spring, it ispossible to adapt the stop mechanism of the present invention topotentiometers having a, wide range of turns. Although some diflicultymight be experienced with one-turn instruments if but one lug was used,the stop mechanism can be adapted to a oneturn instrument by using twoslugs in the place of one. In such a construction, the potentiometerwould be assembled with the one lug engaging the one abutment and thecontact I2 at the one -extreme position relative to the resistanceelement and the second lug would be mounted between the second and thirdturns of the helical spring adjacent to the opposite abutment. Thissecond lug would obviously then move into engagement with the oppositeabutment upon one complete revolution of the shaft. One completerevolution of the shaft would of course move the other lug out ofengagement with its abutment, but one complete revolution of the shaftin the opposite direction would again move this lug back into engagementwith its abutment. In this particular arrangement of the stop mechanism,the number of turns in the helical spring would, of course, beimmaterial.

Although the now preferred embodiments of the present invention havebeen. shown and described herein, it is to be understood that the sameis not to be limited thereto, for it is susceptible to changes in formand detail within the scope of the appended claims.

I claim:

1. A potentiometer of the type described, comprising: a resistanceelement disposed helically about an axis to provide a plurality ofturns, there being a helical space between and bounded by the sides ofadjacent turns; a rotor journalled to turn about the axis of said helix;a contact assembly, including a guide element, disposed in a plane whichis `angled with respect to e, plane perpendicular to said axis, theangle between said planes being substantially equal to the helix angleof said resistance element; said guide element extending into said spaceand engageable with atleast one of adjacent turns defining said space; acontact carried by said contact assembly and spaced from said guideelement; means for mounting said contact assembly to move along saidrotor in a direction substantially parallel to said axis; said guideelement following said helical space as said rotor is turned about saidaxis to advance said contact assembly along said rotor and maintain saidcontact in engagement with said resistance element; and means, separatefrom said contact assembly, for engaging and positively holding saidrotor against continued rotation after said contact assembly has movedsaid contact into engagement with either end of said element, therebylimiting movement of said contact to movement between predeterminedpositions at opposite ends of said element.

2. A potentiometer of the type described, comrising: a resistanceelement providing a plurality of turns extending around an axis, therebeing a space between adjacent turns of the same guration as saidresistance element; a contact assembly, including a guide elementextending into said space between adjacent turns of the element; saidContact assembly providing a con- ,tact spaced from said guide elementand retained in contact with one of said turns by said assembly as thesame moves along said axis; a rotor ournalled to turn about said axis;means for mounting said Contact assembly to move along said rotor toretain said guide element between Said adjacent turns while advancingsaid contact along said resistance element; a pair of V.spaced abutmentsformed on said rotor; shearprising: a resistance element disposedhelically about-an axis to provide a plurality of turns, said elementincluding a core and a resistance wire ywound substantially helicallytherearound to form Vminor turns, the minor turns formed by the"opposite ends of said resistance wire being located inwardly of theopposite ends of said core to-form exposed core portions at the oppositeends of said core; a shaft coaxially mounted for rotational movementrelative to said resistance lelement; contact means engaging saidelement and keyedv to said shaft for rotational movement therewith;means carried by said contact means vextending intermediate adjacentturns of said element and reacting therewith to move said contact meanslongitudinally of said shaft as the same -s rotated, whereby rotation ofsaid shaft moves-said contact means in a helical path alongsaid-element; a pair of abutments longitudinally ispaced'apart on saidshaft; stop means movable `between and into engagement with said abut-`ments; said stop means holding said shaft against -rotation when movedinto engagement with one or 4the other of said abutments; and means forsynchronizing movement of said stop means relative to said abutmentswith movement of said contact means whereby said stop means moves `into,engagement with one or the other of said abutments as said contactmeans moves into engagement with one or the other of the minor vturnsformed by the opposite ends of said resistance wire.

4. A potentiometer of the type described', comprising: a helicalresistance element havinga Ypre-selected number of turns; a shaft; meansfor mounting said shaft coaxially of said element for rotationalmovement relative thereto: a contact adapted to engage said element; acontact carrier mounted to said shaft for rotational movement therewithand longitudinal movement therealong. said carrier extending betweenadjacentturns of said element and reacting therewith to move saidcontact along said resistance element as said shaft is rotated; a pairof. abutments longitudinally spaced apart on said shaft; a lug-,meanscarried by said mounting means for holding said lug against rotationwith said shaft whilepermitting movement of said lug longitudinally ofsaid shaft between and into engagement with one or the other of saidabutments; a helical spring xedly sleeving said shaft and disposedbetween said abutments, said spring deiining a helical passageway havinga number of turns equal to the number of turns of said resistanceelement; the turns of said spring moving said lug longitudinally of saidshaft as the latter is rotated; the movement of said lug into engagementwith one or the other of said abuttation in the direction producingmovement of said lug into engagement with the engaged-abutment.

5. A potentiometer of the type described, com- Iprising: a`helicalresistance element having a preselected number of turns; a shaftcoaxially mounted for rotational movement relative to said element; acontact adapted to engage said element; a contact carrier mounted toSaid shaft'for rotational movement therewith and longitudinal movementtherealong, said carrier extending between adjacent turns of saidelement and reacting therewith to move said contact along saidresista-nce element as said shaft s rotated; a pair of abutmentslongitudinally spaced apart on said shaft; a lug; means for supportingsaid shaft for rotation, including means for mounting said lugs forreciprocal movement'longitudinally of said shaft between and intoengagement with one or the other of said abutments; and a helical springsleeving saidshaft and disposed between said' abutments, said springdef-ning a helical passageway having a number of turns equal to thepreselected number of turns of said resistvance element; said' lug beingmounted for relative lmovement through the turns of said spring as thelatter rotates withsaid shaft, whereby rotation of said shaft moves saidlug longitudinally of said shaft, the eng-agement of said lug with oneor vthe other of said abutments holding said Shaft against'furtherrotation'in the direction producing movement of said lug into engagementwith the engaged abutment.

`6. `A potentiometer of the type described, comprising: a helicalresistance element having a preselected number of turns; a shaftcoaXially mounted for rotational movement relative to said element; acontact adapted to engage said element; a contact carrier transverselymounted toisaid shaft for rotational movement therewith and'longitudinal movement therealong, said carrier extending betweenadjacent turns of said element and reacting therewith to move saidcontact along said resistance element as said shaft mounted forreciprocal movement longitudinally `ofsaid shaft between and intoengagement with one or the other of said abutments; means carriedby'said shaft and disposed between said abutments defining a helicalpassageway having a number of turns equal to the number of turns of saidresistance element; said lug being mounted for relative movement throughthe turns of said passageway as said last named means rotates with saidshaft; the engagement of said lug with one or the other of saidabutments holding said shaft against further rotation in the directionproducing movement of said lug into engagement with the engagedabutment.

7. A potentiometer of the type described, com- `prising: a resistanceelement disposed helically about an axis to provide a plurality of spaceturns; a shaft mounted for rotation about an axis coincident with saidrst named axis; a relatively thin disc of insulating material keyed tosaid shaft for rotational movement therewith and longitudinal movementtherealong; a flexible arm having one end fixed to said disc; and acontact carried adjacent to the free end of said arm and resilientlyheld thereby in engagement with said element; said disc being formedwith a slit radially extending inwardly from the periphery thereof; thefree end of said arm extending through said slit and holding thedefining edges of said slit spaced apart to form the periphery of saiddisc as a helix having a pitch substantially equal to the distancebetween adjacent turns of said resistance element; the periphery of saiddisc extending between adjacent turns of said resistance element andreacting therewith as said disc rotates with said shaft to move saiddisc longitudinally of said shaft and the contact around the turns ofsaid element.

8. A potentiometer of the type described, comprising: an open endedhousing providing an internal cylindrical wall surface; a sheet ofinsulating material coiled to extend around said wall surface with theopposite end portions thereof in overlapping arrangement; a resistanceelement disposed helically about an axis to provide a plurality ofequally spaced turns; means bonding the outer surfaces of said turns tothe inner surface of said coiled sheet engaging said cylindrical wallsurface; means closing the open ends of said housing; means for holdingsaid closing means against displacement from said housing, said holdingmeans securing said coiled sheet to said housing thereby fixing saidresistance element against the movement relative to said housing; anelectrical conductor connected to opposite ends of said resistanceelement, the inner overlapping end portion of said sheet having anopening formed therein through which at least one of said conductors ispassed to longitudinally extend between said overlapping end portions.

9. A potentiometer of the type described, comprising: a resistanceelement disposed helically about an axis to provide a number of turns; ashaft; means for mounting said shaft for rotation about an axiscoincident with said first named axis, a contact assembly mounted tosaid shaft for rotation therewith and longitudinal movement therealong;means for moving said assembly longitudinally of said shaft as the sameis rotated; a Contact mounted to said assembly and engaged with andmovable about the turns of said element as said assembly is moved alongsaid shaft as it rotates therewith; means fixed to said shaft defining apassageway helically arranged about an axis coincident with the axis ofrotation of said shaft, said passageway formed of the same number ofturns as said element; a member slidably mounted in a groove formed insaid shaft-mounting means longitudinally extending relative to saidshaft; said member having a portion thereof disposed within saidpassageway and relatively7 movable therethrough as said shaft isrotated; an abutment fixed to said shaft and disposed at each end ofsaid passageway, revolutions of said shaft equal in number to the turnsof said element moving said member from a position in which the same isengaging one of said abutments to a position in which the same isengaging the other of said abutments while simultaneously moving saidcontact from a predetermined position adjacent one end of said elementto a predetermined position adjacent the other end of said element.

l0. A potentiometer of the type described, comprising: a resistanceelement disposed helically about an axis to provide a number of turnsequally spaced apart; a shaft; means for mounting said shaft forrotation about an axis coincident with said first named axis; a contactassembly mounted to said shaft for rotation therewith and longitudinalmovement therealong; means carried by said assembly extending betweenadjacent turns of said element and reacting therewith to move saidassembly longitudinally of said shaft as the same is rotated; a contactmounted to said assembly and engaged with and movable about the turns ofsaid element as said assembly is moved along said shaft; means on saidshaft dening a helical passageway of the same number of turns as saidelement; a member held against rotation with said shaft and movablelongitudinally of the same; said member having a portion thereofdisposed within said passageway and relatively movable therethrough assaid shaft is rotated; and an abutment fixed to said shaft and disposedat each end of said passageway; movement of said member into engagementwith either one of said abutments by rotation of said shaft holding saidshaft against a continued rotation in the direction producing movementof said member into engagement with an abutment.

1l. A potentiometer of the type described, comprising: a resistanceeiement dispose-d helically about an axis to provide a plurality ofturns, said element including a core and a resistance wire woundsubstantially helically therearound to form minor turns, the minor turnsformed by the opposite ends of said resistance wire being locatedinwardly of the opposite ends of said core to form exposed core portionsat the opposite ends of said core; a shaft coaxially mounted forrotational movement relative to said resistance element; contact meansengaging said element and keyed to said shaft for rotational movementtherewith; means carried by said contact means extending intermediateadjacent turns of said element and reacting therewith to move saidcontact means longitudinally of said shaft as the same is rotated,whereby rotation of said shaft moves said contact means in a helicalpath coincident with the turns of said element; a pair of abutmentslongitudinally spaced apart on said shaft; a lug movable between andinto engagement with one or the other of said abutments; said last namedmeans holding said shaft against rotation when moved into engagementwith one or the other of said abutments; and a helical springsurrounding said shaft between said abutments, said spring defining ahelical passageway of the same number of turns as said resistanceelement and synchronizing movement of said lug relative to saidabutments with movement of said contact means relative to said elementwhereby said lug moves into engagement with one or the other of saidabutments as said Contact means moves into engagement with one or theother of the minor turns formed by the opposite ends of said resistancewire.

l2. A potentiometer of the type described, comprising; a resistanceelement disposed helically about an axis to provide a number of turnsequally spaced apart; a shaft; means for mounting said shaft forrotation about an axis coincident with said first named axis; a contactassembly mounted to said shaft for rotation therewith and longitudinalnovement therealong; means carried by said assembly extending betweenadiacent turns of said element and reacting therewith to move saidassembly longitudinally of said shaft as the same is rotated; a contactmounted to said assembly and engaged with and movable about the turns ofsai-d element as said assembly is moved along said shaft by rotationthereof; a pair of abutments longitudinally spaced apart on said shaft;a helical spring coaxially fixed to said shaft intermediate saidabutments; the coils of said spring defining a helical passageway of thesame number cf turns as said resistance element;

and a lug mounted for sliding movement longitudinally of said shaft, aportion of said lug extending into said passageway, whereby rotation ofsaid shaft moves said lug relatively through said passageway coincidentwith movement of said contact about the turns of said element;engagement of said lug with one or the other of said abutments holdingsaid shaft against rotation, thereby limiting movement of said contactrelative to the turns of said element between predetermined positions atopposite ends of said element.

13. A 'potentiometer of the type described, comprising: a helicalresistance element having a preselected number of turns; a shaftcoaxially mounted for rotational movement relative to said element; acontact adapted to engage said element; a contact carrier transverselymounted to said shaft for rotational movement therewith and.longitudinal movement therealong, said carrier extending betweenadjacent turns of said element Vand reacting therewith to move saidcontact defining a helical passageway of the same number of turns assaid element extending between said surfaces; a lug slidably mountedrelative to said shaft; a portion of said lug extending between adjacentturns of said spring; means for holding said lug against rotationalmovement with said spring as the shaft is rotated to move said Contactalong said element; the turns of said spring as the latter rotates withsaid shaft slidably moving said lug longitudinally of said shaft,movement of said lug relative to the turns of said spring coincidingwith movement of said contact relative to the turns of said element sothat said lug moves into engagement with one or the other of saidabutments as said contact moves into a predetermined position at one endor the other of said element, the spaced helical. `surfaces cooperatingwith said spring to move said lug a distance substantially equal to thewidth of the faces of said abutment during the revolution of said shaftrelatively moving said lug through either of the outer turns of saidpassage-x g way.

14. A potentiometer of the type described, comprising: a helicalresistance element having a preselected number of turns; a shaftcoaxially mounted for rotational movement relative to saidsi element; acontact carrier transversely mounted to said shaft for rotationalmovement therewith and longitudinal movement there along; a contactmounted to said carrier and adapated to engage said element; saidcarrier extending between' adjacent turns of said element and reactingtherewith to move said contact along said resistance element as saidshaft is rotated; said shaft being reduced in diameter to form anannular groove therearound; an abutment formed on each'` lug slidablymounted relative to said shaft; a portion of said lug extending betweenadjacent turns of said spring; means for holding said lug againstrotational movement with said spring as the shaft is rotated to movesaid contact along said element; the turns of said spring as the latterrotates with said shaft slidably moving said lug longitudinally of saidshaft, movement of said lug relative to the turns of said passagewaycoinciding with movement of said contact relative to the turns of saidelement so that said lug moves into engagement with one or the other ofsaid abutments as said contact moves into a predetermined position atone end or the other of said element, the helical surface formed on eachside wall of said groove cooperating with said spring to move said lug adistance substantially equal to the width of the abutment during therevolution of the shaft relatively moving said lug through either of theouter turns of said passageway.

l5. A potentiometer of the type described, comprising: a helicalresistance element having a preselected number of turns; a shaftcoaxially mounted for rotational movement relative to said element; acontact carrier transversely mounted to said shaft for rotationalmovement therewith and longitudinal movement therealong, said carrierextending between adjacent turns of said element and reacting therewithto move said contact along said resistance element as said shaft isrotated; said shaft being reduced in diameter to form an annular groovetherearound; an abutment formed on each of the opposite side walls ofsaid groove; a helical spring defining a helical passageway of the samenumber of turns as said element, said spring coaxially fixed to saidshaft and disposed within said groove to extend between said abutments;a lug slidably mounted relative to said shaft; a portion of said lugextending between adjacent turns of said spring; means for holding saidlug against rotational movement with said spring as the shaft is rotatedto move said Contact along said element; the turns of said spring as thelatter rotates with said shaft slidably moving said lug longitudinallyof said shaft, movement of said lug relative to the turns of saidpassageway coinciding with movement of said Contact relative to theturns of said element so that said lug moves into engagement with one orthe other of said abutments as said contact moves into a predeterminedposition at one end or the other of said element;

16. An instrument of the type described, comprising: a shaft; means formounting said shaft for rotational movement about the longitudinal axisthereof; means operatively connected to said shaft and moved thereby asthe same is rotated; and means for limiting rotational movement of saidshaft to a predetermined number of revolutions thereby preselectivelylimiting movement of said last named means, said limiting meansincluding means on said shaft defining a helical passageway having turnsof the same number as said predetermined number, a member held againstrotation with said shaft and movable longitudinally of the same, saidmember having a portion thereof disposed Within said passageway andrelatively movable through one complete turn thereof upon each completerevolution of said shaft, and an abutment fixed to said shaft anddisposed at each end of said passageway, movement of said member intoengagement with either one of said abutments by rotation of said shaftholding said shaft against continued rotation in the direction producingmovement of said .member into engagement with an abutment.

17. An instrument of the type described, comprising: a shaft; means formounting said shaft for rotational movement about the longitudinal axisthereof; means operatively connected to said shaft and moved thereby asthe same is rotated; and means for limiting rotational movement of saidshaft to a predetermined number of revolutions, thereby preselectivelylimiting movement of said last named means between alternate limitpositions, said limiting means including a pair of abutmentslongitudinally spaced apart on said shaft, a helical spring surroundingsaid shaft between said abutments, said spring defining a helicalpassageway having turns of the same number as said predetermined number,a lug mounted between the coils of said spring and movable through thepassageway defined thereby upon rotation of said shaft and intoengagement with one or the other of said abutments depending upon thedi- 18 rection of rotation of said shaft, the helical passageway formedby said spring synchronizing movement of said lug relative to saidabutments with movement of the means operatively connected to saidshaft, whereby said lug moves into engagement with one or the other ofsaid abutments as Said means is moved into one or the other of its limitpositions.

WILLIAM D. VAN DYKE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,606,153 Douglas Nov. 9, 19262,361,010 Cary et al Oct. 24, 1944 2,371,159 Erb Mar. 13, 1945 202,473,048 Beckman June 14, 1949

